1. Field of the Invention
This invention relates to the field of automatic transmission control, particularly to electronic automatic transmission control.
2. Description of the Prior Art
It is conventional practice in automatic transmission control that scheduled gear ratio changes, both upshifts and downshifts, occur automatically on the basis of current vehicle speed and engine throttle position. When throttle position is large, gear shifts occur at higher vehicle speeds, and therefore at higher engine rpm.
Engine spark advance, air-fuel ratio, and other operating parameters are controlled electronically so that engine power meets the expectations of the vehicle operator, represented by commands produced by an on-board computer in response to driver input and external variables.
Certain devices for controlling vehicle subsystems intentionally alter the magnitude of engine torque and engine power from those corresponding to the engine throttle position in order to accomplish the function of those devices. For example, a traction assist system, which modifies operation of an antilock brake system to selectively hold and release the drive wheels of a vehicle to overcome problems associated with wheel slippage, intentionally reduces engine torque for a period in order to reduce or eliminate wheel slippage, although the vehicle operator, concurrently with the engine torque reduction, may demand higher torque than that permitted by the traction assist device by depressing the engine throttle pedal.
A potential conflict results because automatic transmission gear ratio changes occur on the basis of actual throttle position, represented by the extent to which the throttle pedal is depressed, yet the traction assist device reduces engine torque to a lower magnitude than that corresponding to the actual throttle position.
Accordingly, in a vehicle accelerating from a standing start on an icy surface, or another surface where wheel slippage occurs, the traction assist system decreases engine torque by producing signals that modify engine operation such as by altering engine spark timing, air-fuel ratio, or other engine parameters. Under these circumstances, the vehicle accelerates slowly due to the low surface friction, but driver demand, as measured by throttle position, may cause shifts to occur at high engine rpm. This condition produces high engine noise and relatively harsh gear ratio changes.